Seamless Stainless Steel Pipe Manufacturing Process

What Is a Seamless Stainless Steel Pipe?

HAOMEI seamless stainless steel pipe is an important industrial pipe widely used in petrochemical, energy and power, shipbuilding, mechanical equipment, food processing, and pharmaceutical industries.

Unlike welded pipes, seamless stainless steel pipes have no weld seam throughout the entire pipe structure, which provides higher structural strength, superior pressure resistance, and better reliability. They are particularly suitable for applications involving high temperature, high pressure, and highly corrosive environments.

Development History of Seamless Stainless Steel Pipes

Stainless steel seamless pipes are made by piercing steel ingots or solid tube blanks to form a rough tube, which is then hot-rolled, cold-rolled, or cold-drawn. The specifications of stainless steel seamless pipes are expressed in millimeters as outer diameter * wall thickness. Hot-rolled seamless pipes generally have an outer diameter greater than 32mm and a wall thickness of 2.5-200mm. Cold-rolled seamless steel pipes can have an outer diameter down to 6mm and a wall thickness down to 0.25mm. Thin-walled pipes can have an outer diameter down to 5mm and a wall thickness less than 0.25mm. Cold rolling offers higher dimensional accuracy than hot rolling.

Based on different production methods, they are mainly divided into:

• Hot-rolled seamless stainless steel pipes

• Cold-rolled seamless stainless steel pipes

• Cold-drawn seamless stainless steel pipes

Hot rolling is suitable for producing large-diameter, thick-walled pipes, while cold rolling or cold drawing is mainly used for producing high-precision, small-diameter pipes.

Based on the metallographic structure of stainless steel, it can be divided into:

Semi-ferritic/semi-martensitic stainless steel pipes, martensitic stainless steel pipes, austenitic stainless steel pipes, austenitic-ferritic stainless steel pipes, etc.

Stainless Steel Seamless Pipe Production Process

Typical manufacturing flow:

Round bar preparation → Heating → Hot piercing → Head cutting → Pickling → Grinding → Lubrication → Cold rolling → Degreasing → Solution heat treatment → Straightening → Pipe cutting → Pickling → Final inspection

Detailed Explanation of the Seamless Stainless Steel Pipe Manufacturing Process

Hot Rolled Seamless Stainless Steel Pipe Production Process

1. Raw Material Preparation

The production of seamless stainless steel pipes usually begins with high-quality stainless steel round billets.

Common materials include:

• 304 / 304L stainless steel

• 316 / 316L stainless steel

• 321 stainless steel

• 347 stainless steel

• Duplex stainless steel 2205

Before entering the production line, raw materials undergo rigorous testing, including sampling and analysis. The chemical composition is checked to ensure it matches the values on the quality assurance certificate. Advanced testing equipment such as metallurgical microscopes with imaging capabilities and continuous zoom stereomicroscopes are also used for inspection. Low-temperature impact tests are conducted to verify that the toughness meets national standards, ensuring the raw materials comply with production standards.

Finally, production begins based on product and process drawings provided by the technical department.

2. Heating and Piercing

The stainless steel billets are first heated in a furnace to 1100–1250°C, giving the material good plasticity.

After heating, the billets undergo high-pressure water descaling and are sent to the piercing mill (cross-roll piercing machine or Mannesmann piercing mill).

Under strong compressive forces, conical rollers rotate and push the billet forward while a mandrel pierces the center, transforming the solid billet into a hollow tube called a hollow shell.

During the piercing process, the billet must continuously rotate and move forward.

3. Rolling

The hollow tube enters a three-roll mill or continuous rolling mill, where it is rolled repeatedly with a mandrel.

This process reduces wall thickness, increases length, and gradually achieves the required outer diameter and wall thickness.

4. Sizing and Reducing

The tube then passes through sizing mills or reducing mills, which precisely control the final outer diameter, eliminate ovality, and improve dimensional accuracy.

5. Cooling and Heat Treatment

After hot rolling, the pipes are air-cooled or water-cooled.

Depending on the material grade, solution annealing may be required. For example, austenitic stainless steel is heated to 1050–1100°C and then rapidly cooled, which eliminates internal stress, stabilizes the microstructure, and improves corrosion resistance.

6. Straightening and Surface Treatment

Straightening:
During rolling or drawing processes, pipes may become slightly bent. Straightening machines are used to ensure proper straightness.

Surface treatment:
Pickling removes oxide scale and improves surface cleanliness. Further polishing can be applied if necessary.

7. Quality Inspection and Testing

Non-destructive testing: hydrostatic/pneumatic testing, eddy current/ultrasonic testing to ensure no cracks or leaks.

Final cutting, marking, and warehousing.

Cold Rolled / Cold Drawn Seamless Stainless Steel Pipe Production Process

Cold drawing or cold rolling uses hot-rolled hollow tubes as raw materials and processes them at room temperature through powerful drawing or rolling operations.

This method is mainly used to produce small-diameter, high-precision pipes with smooth surfaces.

1. Hollow Tube Pre-treatment

End reduction to facilitate drawing through the die

Annealing at around 900°C to eliminate residual stress and restore ductility

Pickling and lubrication to remove oxide scale and reduce friction during drawing

2. Multi-Pass Cold Drawing or Cold Rolling

Cold drawing:
The tube passes through a fixed die and is pulled by a drawing machine, reducing the outer diameter and wall thickness while extending the length.

Cold rolling (Pilger mill):
A reciprocating rolling process using shaped dies, providing extremely high precision (wall thickness tolerance up to ±3%), especially suitable for thin-wall precision tubes.

Intermediate annealing is required between passes to prevent cracking caused by work hardening.

3. Finished Product Processing

Final heat treatment (solution/stress-relief annealing), straightening, pickling/polishing, non-destructive testing, length cutting, and warehousing.

Comparison of the Two Manufacturing Processes

Key Quality Control Points

• Chemical composition
  Strict control of carbon, sulfur, and phosphorus to ensure corrosion resistance and plasticity.

• Temperature control
  Heating, piercing, and annealing temperatures significantly affect microstructure and performance.

• Deformation control
  Reduction rates must match material properties to prevent cracking or uneven microstructure.

• Non-destructive testing
  Full inspection throughout the process ensures no internal defects.

• Surface treatment
  Proper removal of oxide scale and acid residue prevents corrosion risks.

Seamless Stainless Steel Pipe Standards

Applications of Seamless Stainless Steel Pipes

Due to their excellent performance, seamless stainless steel pipes are widely used in many industries.

• Oil and Gas Industry : Used for transporting high-temperature and high-pressure oil and gas media.

• Chemical Industry : Used for conveying corrosive chemical fluids.

• Power Industry : Applied in boiler pipelines and heat exchanger equipment.

• Shipbuilding Industry : Used in seawater cooling systems and ship piping systems.

• Food and Pharmaceutical Industry : Used for sanitary fluid transportation pipelines.